Triiodide-in-Iodine Networks Stabilized by Quaternary Ammonium Cations as Accelerants for Electrode Kinetics of Iodide Oxidation in Aqueous Media
Hyeonmin Kim, Kyungmi Kim, Jungju Ryu, Sehyeok Ki, Daewon Sohn, Junghyun Chae, Jinho Chang
Abstract
The Zn–polyiodide redox flow battery is considered to be a promising aqueous energy storage system. However, in its charging process, the electrode kinetics of I– oxidation often suffer from an intrinsically generated iodine film (I2–F) on the cathode of the battery. Therefore, it is critical to both understand and enhance the observed slow electrode kinetics of I– oxidation by an electrochemically generated I2–F. In this article, we introduced an electrogenerated N-methyl-N-ethyl pyrrolidinium iodide (MEPI)–iodine (I2) solution, designated as MEPIS, and demonstrated that the electrode kinetics of I– oxidation were dramatically enhanced compared to an I2–F under conventional electrolyte conditions, such as NaI. We showed that this result mainly contributed to the fast electro-oxidation of triiodide (I3–), which exists in the shape of a I3–-in-I2 network, [I3–·(I2)n]. Raman spectroscopic and electrochemical analyses showed that the composition of electrogenerated MEPIS changed from I3– to [I3–·(I2)n] via I5– as the anodic overpotential increased. We also confirmed that I– was electrochemically oxidized on a MEPIS-modified Pt electrode with fast electrode kinetics, which is clearly contrary to the nature of an I2–F derived from a NaI solution as a kinetic barrier of I– oxidation. Through stochastic MEPIS–particle impact electrochemistry and electrochemical impedance spectroscopy, we revealed that the enhanced electrode kinetics of I– oxidation in MEPIS can be attributed to the facilitated charge transfer of I3– oxidation in [I3–·(I2)n]. In addition, we found that the degree of freedom of I3– in a quaternary ammonium-based I2–F can also be critical to determine the kinetics of the electro-oxidation of I–, which is that MEPIS showed more enhanced charge-transfer kinetics of I– oxidation compared to tetrabutylammonium I3– due to the higher degree of freedom of I3–.